You’ll want to make sure your tokenized sentences fit in the context length so they don’t get truncated but aside from that you’ll probably just want to try different things and see what works. There’s no rule of thumb that I’m aware of for this problem. If you have some ground truth for what the correct result of a vector search should be then evaluation is pretty straightforward, otherwise you might have to do A/B tests on users or even just eyeball it.
It’s also pretty standard practice to have your passages overlap so that the bit of text you care about for a particular search doesn’t get cut in half just in case you weren’t aware already.